1. Field of the Invention
The present invention relates to valve crosshead assemblies which carry valves of an internal combustion engine, and which are acted upon by the valve rocker levers of the valve drive train. The present invention also relates to the use of ceramic materials to provide wear-resistant contact surfaces for internal combustion engine components.
2. Description of Related Art
The harsh operating conditions encountered in an internal combustion engine, particularly the high temperatures and high pressures, cause engine components to wear rapidly. Mechanically driven actuators and actuating components are especially susceptible to wear in this environment. Consequently, the materials used for producing actuating engine components should provide good mechanical strength, thermal stability and wear resistance. While metals have typically been used to form such components, ceramics, such as zirconia, silicon nitride, silicon carbide and the like, have been found to exhibit excellent mechanical strength, thermal stability and improved wear resistance relative to metals. As a result, ceramics are increasingly being used as structural materials for components of gas turbine engines and diesel engines.
However, even though ceramics can provide highly wear-resistant engine components, they are generally hard and brittle and lack the formability and workability of the metals which are conventionally applied to low cost precision engine components. Furthermore, while components formed from a ceramic element and a metal element have been proposed to overcome these limitations, and components of this type that can be useful as internal combustion engine components are available, composite components formed of metal and ceramic elements are not without their own problems. The low thermal expansion and tensile strength properties of structural ceramics relative to metals, in general, make formation of a secure connection between these two elements difficult to achieve. Presently, each element is machined to produce tolerances that are not only sufficiently precise to ensure the retention of the ceramic element in the metal element during engine operation, but which also allow for the differential thermal expansion of the ceramic and the metal, and limit tensile stresses in the ceramic.
In the case of the cylinder head valve drive train of an internal combustion engine, a push rod used to transmit movement of a cam to a valve rocker lever, which acts on a crosshead to which cylinder head valves are mounted, is known to be formed of a composite metal and ceramic component in which a ball and socket component is joined to a hollow tube using interference fit methods as is disclosed in U.S. Pat. No. 4,794,894 to Gill, and U.S. Pat. No. 4,806,040 to Gill et al., both assigned to Cummins Engine Company, Inc., assignee of the present invention. U.S. Pat. No. 4,848,286 to Bentz, also assigned to Cummins Engine Co., discloses the use of an external metal connector for joining ceramic and metal components of a pivot rod.
Additionally, a lightweight metal rocker arm having a wear-resistant, cam-engaging pad formed of a ceramic material is disclosed in U.S. Pat. No. 4,995,281 to Allor et al. The ceramic pad is, preferably, integrally joined to the rocker arm during casting of the rocker arm but it is also indicated that it could be affixed after forming by such other techniques as adhesive bonding, brazing, or interference fitting.
In Sato et al. U.S. Pat. No. 4,838,218, a ceramic valve is joined to a metal spring retainer via a tapered metal cotter. The metal cotter is tightly engaged between the stem of the ceramic valve and a tapered annular inner wall of the retainer, and to minimize the chance of resulting stress concentrations in the ceramic valve stem, a stress relief coating or layer is applied to the surface of the cotter which engages the valve stem.
Apart from uses in a cylinder valve drive train, use of an interference fit to secure a ceramic component to a metal component to form a composite structure useful in an internal combustion engine is also shown in U.S. Pat. No. 4,366,785 to Goloff et al., for example. This patent discloses a tappet for an internal combustion engine with a ceramic wear resistant insert maintained within the annular metal rim of the main body of the tappet by an interference fit. The wear resistant insert is formed to be slightly larger in diameter than the diameter of the recess into which it is fitted. The ceramic insert is forced into the recess under sufficient pressure to press fit it in the tappet main body. The insert is not required to be sized to fit exactly within the recess in the tappet, but must be slightly larger than the recess. However, to provide a secure interference fit without damaging the metal or ceramic components, each must still be formed to close tolerances.
With respect to a piston for an engine cylinder, U.S. Pat. No. 4,325,647 to Maier et al. discloses a method of securing a ceramic wear resistant element to a metal element using a separate connecting element formed from an insulating resilient body of a ceramic material by which thermally induced differences between the ceramic and metal. structures are equalized, and contact stress in the operating state is limited. The insulating body positively connects the ceramic and metallic elements and operates effectively to secure these elements when it has specific physical characteristics, for example, a thermal conductivity of 0.02 to 0.25 W/cmK at a temperature difference between the ceramic and the metallic structural elements of about 100.degree. to 1500.degree. C. and an elastic modulus of about 5000 and 150,000 N/mm.sup.2. This composite, however, is not intended to be used for a sliding friction interface between mechanically driven valve actuating components.
Also, external connectors have been proposed for joining a ceramic element to a metal element of a piston. U.S. Pat. No. 4,833,911 to Haahtela discloses a ceramic piston ring carrier held in place on a metal piston by casting in or with a locking ring to improve force transmission and frictional conditions between the piston and the cylinder. However, this patent does not suggest how that arrangement described therein could be used to secure a ceramic element to a metal element to form a wear-resistant interface between engine valve actuating components where sliding friction is a significant factor.
Lastly, commonly assigned, U.S. Pat. No. 5,279,211, two of the inventors of which are the inventors of the present invention, internal combustion engine actuator or actuating components, such as compression brake master pistons and hydraulic tappet sliding cam followers, are formed of a composite structure of metal and ceramic members which minimizes tensile ceramic loads and accommodates differences in thermal expansion characteristics between the metal member and the ceramic member without reliance on precise physical control of the dimensions of either member. The composite component includes a mechanical retainer which allows a loose fitting relationship between the metal and ceramic members. The ceramic member is secured within a receiving bore in the metal member by the retainer in a manner which eliminates the need for precise machining of the ceramic and metal members. The metal member may be configured to accept either an internal or an external mechanical retainer element.
However, despite the extensive use in the prior art of engine components formed of united ceramic and metal elements, no arrangement has yet been devised in which a ceramic sliding friction wear element is provided for a valve crosshead that is engaged by a valve rocker lever, and in particular, in a manner directed to the specific needs of and which will be sufficiently reliable when used for a valve crosshead. In particular, no metal valve crosshead has been provided with a ceramic insert which will not only reduce frictional wear but also energy loss from contact by the rocker lever and friction induced side loads, while still being capable of commercially feasible, inexpensive, high volume production.